Document feature flags

[joshka]

Summary

• move the feature documentation into the crate-level getting started section
• match the feature wording and grouping with palette/README.md
• keep the original feature definitions unchanged in Cargo.toml

Why

This follows the same documentation pattern used by palette_math, where the crate docs and README
can be edited in tandem without introducing another source of truth for feature descriptions.

Closes #415.

Validation

• cargo +nightly docs-rs -p palette

[joshka]

[codspeed-hq]

Merging this PR will not alter performance

✅ 47 untouched benchmarks

---

_{Comparing joshka:codex/document-feature-flags (caa8a92) with master (60db153)}

[Ogeon]

Hi, thanks for taking the time to do this! I should have done something similar long ago but it slipped my mind.

As much as I appreciate the help and effort, I would like the wording and grouping to match the readme and for it to be part of a "getting started" section up top. If you look at what I did for palette_math, you can see that it's essentially a copy of that part of the readme. That makes it easy to edit them in tandem. In fact, I don't mind going without document-features since it doesn't completely remove the duplication. Copy-pasting is simple too. 😅

Second, please don't change the feature set, even though some may seem useless. It's a breaking change after all and potentially unnecessary churn.

Also please follow the template for closed issues. Those keywords will close the issue automatically, and I'm also using a script for reading them when generating the release notes.

Oh and ignore the miri error, it's just fallout from floating point number randomization they added to emulate platform differences. I just got to relax the precision requirement in that test.

[Ogeon]

The miri error should hopefully be fixed now if you rebase this on top of master.

[Ogeon]

Ah, great! Thank you!